This invention relates to fuel cells and, more particularly, to catalyst loadings for solid polymer electrolyte fuel cells. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Fuel cells are energy conversion devices presently being considered as one alternative to internal combustion engines. One type of fuel cell uses a solid polymer electrolyte (SPE) membrane, or proton exchange membrane, to provide ion exchange between the cathode and anode electrodes. Gaseous fuels may be used within the fuel cell, particularly hydrogen (H.sub.2) and oxygen (O.sub.2), where the electrodes are formed of porous conductive materials, e.g., woven graphite, to enable the fuel to disperse over the face of the SPE.
SPE fuel cells offer many advantages over liquid electrolyte fuel cells, including greater ultimate power densities, lower operating temperatures, and longer operating lifetimes. SPE materials are also generally resistant to corrosion and easy to incorporate into fuel cell structures. However, the anode and cathode half-cell reactions, H.sub.2 and O.sub.2 reactions, respectively, require catalysts to proceed at useful rates. As described in U.S. Pat. No. 4,876,115, issued Oct. 24, 1989, and incorporated herein by reference, catalyst materials were first incorporated by hot pressing the materials directly into the surface of the SPE membrane. Useful current densities in conventional SPE fuel cells were achieved only with high catalyst loadings, e.g., 4 mg Pt/cm.sup.2. Since the catalyst materials are from the platinum group, with platinum being the preferred catalyst, these SPE fuel cells (herein referred to as GE/HS-UTC-type fuel cells) have not been cost competitive with other energy sources.
The '115 patent is directed to reducing the required platinum loading where the platinum is provided as platinum particles supported on carbon pieces, referred to as a supported catalyst (Pt-C), on a carbon cloth or carbon paper electrode substrate bound together by a hydrophobic component, such as polytetrafluorethylene (PTFE). The catalyzed sides of the carbon electrodes are impregnated to a depth of about 10 .mu.m with a solubilized form of the SPE to increase the access of the electrolyte to the supported platinum catalyst within the impregnated layer. Indeed, catalyst loadings down to 0.35 mg/cm.sup.2 of SPE area are reported to provide performance equivalent to conventional fuel cell catalyst loadings of 4 mg/cm.sup.2.
The platinum catalyst is not efficiently utilized in the prior art structures. It is difficult to match the impregnation depth of the SPE with the erratic thickness of a typical catalyst layer. This results in areas that are not fully impregnated and other areas where the SPE material extends deeper into the electrode than the catalyst layer and impedes gas diffusion through the electrode. It is also difficult to obtain a high loading, i.e., a high weight percent, of the SPE ionomer to maximize contact between catalyst sites and the ionomer when a impregnation technique is used to introduce the ionomer. Further, the hydrophobic binder blocks proton and oxygen access to catalyst sites in cathode electrodes.
Another problem with prior art fuel cells is differential swelling between the SPE and the catalyst layer arising from the differing hydration characteristics between the hydrophilic SPE membrane and the carbon-based electrode structure. Delamination can occur between the SPE membrane and the electrode with a resulting discontinuity in the ion path and decreased cell longevity.
These problems are addressed by the present invention and a catalyst layer is provided adjacent a fuel cell SPE that is hydrophilic, contains substantially no cavities, is uniformly thin, and contains a uniform ratio of binder ionomer to supported catalyst.
Accordingly, it is an object of the present invention to provide a SPE fuel cell with relatively low supported catalyst loadings with no reduction in performance.
It is another object to provide uniform continuity of electronic and ionic paths about all of the catalyst sites.
Still another object is to provide a uniform dispersion of the supported catalyst layer in the binder layer.
One other object is to improve the bonding between the SPE membrane and the catalyst layer.
Yet another object is to provide a thin catalyst layer for adequate oxygen transport to all the catalyst sites through the ionomer binder material.
Another object is to increase the weight fraction of the SPE ionomer with the catalyst layer to improve the efficiency of the catalyst.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.